To appropriately design a service area for such a wireless communications system as a cellular phone network system or wireless LAN system, it is necessary to properly understand the radiowave environment characteristics obtainable when a base station to be designed (inclusive of access points in the wireless LAN system as well as the base station in the cellular phone network system) is installed at a specified location and with specified parameters. A radiowave propagation simulator is used to achieve the above. The appropriate service area can be designed by determining the installation location and parameters for the base station while assessing the signal-receiving electric field strength, delay spread, and other factors observable at any observing point using the radiowave propagation simulator.
Roughly speaking, radiowave propagation simulation employs a statistical method or a deterministic method. The statistical method gives a propagation loss estimating formula with the distance from a base station to a specified location, a frequency, and other values, as arguments, and empirically imparts parameters based on large volumes of actually measured propagation loss data, to the estimating formula. The average situation of terrestrial objects around a transmitting point or a receiving point can also be used to correct estimated data. The average situation of terrestrial objects refers to, for example, a building area occupancy ratio or average building height. Examples of the statistical method include the Okamura-Hata model, the Sakagami model, etc. Details of the Okamura-Hata model and the Sakagami model are disclosed in Non-Patent Document 1.
In contrast, the deterministic method allows for incorporating any impacts of peripheral objects faithfully when estimating the status of the radiowave propagation ranging from the transmitting point to the observing point. For example, if the deterministic method is used to estimate the status of the radiowave propagation originating from a base station disposed outdoors, the buildings, terrains, and other objects affecting the radiowave propagation are modeled using building map data and altitude map data and the radiowave propagation status is estimated while faithfully considering the impacts of the objects, such as reflection, transmission, and the like. Examples of a deterministic method include methods such as ray tracing. Ray tracing is a method of calculating propagation losses and the amounts of delay, by looking upon antenna-radiated radio waves as a cluster of radio rays, regarding each of the rays as propagating while repeating reflection and transmission in geometric-optical fashion, and combining the rays reaching the observing point.
The radiowave environment data to be given by radiowave propagation simulation, however, will contain estimation errors concerning the radiowave environment data obtained by actual installation of the base station under the same conditions as those of simulation. In the statistical method, for example, a statistical formula that has been empirically derived in a certain environment is applied to that of another environment to allow for the impacts of surrounding objects upon the radio wave, so the environmental differences may cause significant estimation errors. In the deterministic method, although the impacts of the objects are allowed for deterministically and can therefore be estimated more accurately than in the statistical method, estimation errors will also occur if the building map data or layout data used as object information is erroneous.
A method for correcting estimated radiowave environment data using the data actually measured at a limited area (e.g., on a trunk road) in an area surrounding a base station is disclosed as a method for reducing the estimation errors mentioned above. Patent Document 1, for example, discloses a method for simulating a radiowave propagation status by storing measured data on desired-wave receiving electric power, then calculating separate correction values for each of specified directions of a propagation curve based upon a statistical method, from any differences between the measured data and the propagation curve, next using the calculated correction values to correct the propagation curve, and finally, using the corrected propagation curve. For example, the Okamura-Hata model is used as the propagation curve. In addition, a building area occupancy ratio, for example, is corrected to correct the propagation curve. A correction value for the building area occupancy ratio is calculated for each specified angle (e.g., every two degrees) in all directions around the wireless base station, and is intended to simulate the propagation status of radio waves for each specified angle using the corrected propagation curve.
Another method for correcting estimated radiowave environment data using measured data is disclosed in Patent Document 2. In the method of Patent Document 2, differences between the propagation losses that have been measured at a plurality of points, and the propagation losses that have been arithmetically estimated using a propagation model, are calculated and virtual height values for each terrestrial object are adjusted so that the differences are decreased. The virtual height is one of the functions given in the radiowave propagation simulator “NetPlan” (registered trademark) developed by the applicant of Patent Document 2, and means the virtual height assumed for each object feature present on the earth surface.
The methods disclosed in Patent Documents 1 and 2 differ in that whereas one corrects a building area occupancy ratio, the other corrects virtual height. However, both methods are intended to perform corrections upon the area around the base station that corresponds to the direction in which measured data exists. As a result, radiowave environment data that incorporates the corrections is given to the area of the same direction as that in which the measured data exists.